Add ListBuilder (#1669)

A ListBuilder allows creating large lists efficiently

js/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@attic/noms",
-  "version": "41.4.0",
+  "version": "41.5.0",
   "description": "Noms JS SDK",
   "repository": "https://github.com/attic-labs/noms",
   "main": "dist/commonjs/noms.js",

js/src/list-test.js

@@ -26,7 +26,7 @@ import {
 } from './test-util.js';
 import {MetaTuple, newListMetaSequence} from './meta-sequence.js';
 import {invariant, notNull} from './assert.js';
-import List from './list.js';
+import List, {ListWriter} from './list.js';
 import {equals} from './compare.js';
 
 const testListSize = 5000;
@@ -492,4 +492,55 @@ suite('Diff List', () => {
 
     assert.strictEqual('Load limit exceeded', exMessage);
   });
+
+  test('ListWriter', async () => {
+    const values = intSequence(15);
+    const l = new List(values);
+
+    const w = new ListWriter();
+    for (let i = 0; i < values.length; i++) {
+      w.write(values[i]);
+    }
+
+    w.close();
+    const l2 = w.list;
+    const l3 = w.list;
+    assert.isTrue(equals(l, l2));
+    assert.strictEqual(l2, l3);
+  });
+
+  test('ListWriter close throws', () => {
+    const values = intSequence(15);
+    const w = new ListWriter();
+    for (let i = 0; i < values.length; i++) {
+      w.write(values[i]);
+    }
+    w.close();
+
+    let ex;
+    try {
+      w.close();  // Cannot close twice.
+    } catch (e) {
+      ex = e;
+    }
+    assert.instanceOf(ex, TypeError);
+  });
+
+  test('ListWriter write throws', () => {
+    const values = intSequence(15);
+    const w = new ListWriter();
+    for (let i = 0; i < values.length; i++) {
+      w.write(values[i]);
+    }
+    w.close();
+
+    let ex;
+    try {
+      w.write(42);  // Cannot write after close.
+    } catch (e) {
+      ex = e;
+    }
+    assert.instanceOf(ex, TypeError);
+  });
+
 });

js/src/list.js

@@ -12,14 +12,18 @@ import {IndexedSequence, IndexedSequenceIterator} from './indexed-sequence.js';
 import {diff} from './indexed-sequence-diff.js';
 import {getHashOfValue} from './get-hash.js';
 import {invariant} from './assert.js';
-import {MetaTuple, newIndexedMetaSequenceBoundaryChecker,
-  newIndexedMetaSequenceChunkFn} from './meta-sequence.js';
+import {
+  MetaTuple,
+  newIndexedMetaSequenceBoundaryChecker,
+  newIndexedMetaSequenceChunkFn,
+} from './meta-sequence.js';
 import {sha1Size} from './hash.js';
 import Ref from './ref.js';
 import {getValueChunks} from './sequence.js';
 import {makeListType, makeUnionType, getTypeOfValue} from './type.js';
 import {equals} from './compare.js';
 import {Kind} from './noms-kind.js';
+import SequenceChunker from './sequence-chunker.js';
 
 const listWindowSize = 64;
 const listPattern = ((1 << 6) | 0) - 1;
@@ -154,3 +158,41 @@ function clampIndex(idx: number, length: number): number {
 
   return idx < 0 ? Math.max(0, length + idx) : idx;
 }
+
+type ListWriterState = 'writable' | 'closed';
+
+export class ListWriter<T: Value> {
+  _state: ListWriterState;
+  _list: ?List<T>;
+  _chunker: SequenceChunker<List<T>, T, ListLeafSequence<T>>;
+
+  constructor() {
+    this._state = 'writable';
+    this._chunker = new SequenceChunker(null, newListLeafChunkFn(),
+        newIndexedMetaSequenceChunkFn(Kind.List, null), newListLeafBoundaryChecker(),
+        newIndexedMetaSequenceBoundaryChecker);
+  }
+
+  write(item: T) {
+    assert(this._state === 'writable');
+    this._chunker.append(item);
+  }
+
+  close() {
+    assert(this._state === 'writable');
+    this._list = this._chunker.doneSync();
+    this._state = 'closed';
+  }
+
+  get list(): List {
+    assert(this._state === 'closed');
+    invariant(this._list);
+    return this._list;
+  }
+}
+
+function assert(v: any) {
+  if (!v) {
+    throw new TypeError('Invalid usage of ListWriter');
+  }
+}

js/src/noms.js

@@ -23,7 +23,7 @@ export {
 export {encodeNomsValue} from './encode.js';
 export {invariant, notNull} from './assert.js';
 export {isPrimitiveKind, Kind, kindToString} from './noms-kind.js';
-export {default as List, ListLeafSequence} from './list.js';
+export {default as List, ListWriter, ListLeafSequence} from './list.js';
 export {default as Map, MapLeafSequence} from './map.js';
 export {default as Set, SetLeafSequence} from './set.js';
 export {IndexedSequence} from './indexed-sequence.js';

js/src/sequence-chunker.js

@@ -60,7 +60,7 @@ export function chunkSequenceSync<C: Collection, S>(
   return chunker.doneSync();
 }
 
-export default class SequenceChunker<C: Collection, S, U:Sequence> {
+export default class SequenceChunker<C: Collection, S, U: Sequence> {
   _cursor: ?SequenceCursor<S, U>;
   _isOnChunkBoundary: boolean;
   _parent: ?SequenceChunker<C, MetaTuple, MetaSequence>;